Many ideas for improvements in the world's energy usage focus on increasing the efficiency of existing types of engines. Most heat engines are limited in their efficiency by the theoretical efficiency of the Carnot cycle, which requires an increase in operating temperature in order to increase operating efficiency. A typical application for a heat engine is to generate electricity by boiling water to create superheated steam and using the expansion of the steam to drive a turbine attached to a generator. This works very well if two temperature reservoirs can be created with a large temperature difference between them to facilitate a large expansion ratio of the superheated steam as it cools. Other gaseous working mediums having different specific heats and boiling points may be used, but in all cases the maximum efficiency of the heat engine is defined by the increase in temperature which can be achieved in the heat source over the temperature of the heat sink.
If, however, one wishes to harvest a source of thermal energy with a low temperature relative to any available cooling reservoir, then low efficiencies and low power output must be accepted when using currently available heat engine technologies. Accordingly, additional methods of harvesting energy from relatively low temperature sources of thermal energy are desirable.
Some heat engines using phase change materials, such as Nickel-Titanium alloys known as nitinol, have been designed in which the engine efficiency does not depend on the difference in temperature between the heat source and the heat sink. These engines are theoretically capable of utilizing relatively low-temperature sources of heat. These engines, however, tend to be rather inefficient and do not take advantage of the full phase change expansion that nitinol undergoes. Many of the existing designs do not fully insulate the heat source from the heat sink and therefore do not efficiently use the available heat. Accordingly, there is a need for a more efficient engine that utilizes a phase change material.